Injection molding--the process of injecting a quantity, or shot, of molten plastic into a mold--is today one of the world's dominant forms of plastic article manufacture. However, a product uniformity problem plagues this process because of the inability to control perfectly the quantity of material injected into the mold. This imperfection is caused primarily by the failure of a non-return valve, found on most injection molding machines, to close in a consistent, repeatable manner during the injection step.
Non-return valves allow molten plastic to flow from the feed screw to an accumulation volume. Injection occurs when the accumulation volume is full. During injection, the entire valve assembly strokes forward, forcing the molten material from the accumulation volume into the mold. Non-return valves are designed to shut during the injection process, preventing back flow through the valve. A review of the prior art illustrates two primary methods of sealing against material back flow during the injection step: a ring-type shut-off valve or a ball-type check valve. With these methods, as the injection ram strokes forward, a ball or piston is forced against a seat or a tapered ring is forced against another ring with a complementary taper. With a ball-type valve, plastic back leakage over the ball creates a pressure drop across the ball. This pressure drop becomes the primary force closing the valve. However, any back leakage variation before the valve closes causes a variation in the quantity of plastic in the accumulation volume.
Ring-type valves use a cylindrical ring rather than a ball to close the valve. FIGS. 1a and 1b illustrate an example of a prior art ring valve. This ring fits outside the valve body, through which the molten material flows. A barrel contains the entire assembly. The ring's upstream and downstream travel are limited by upstream and downstream retainers. The ring's upstream face is tapered, or angled a certain amount from perpendicular to the valve's longitudinal axis. When open (FIG. 1a), material flows over the ring's upstream stop, between the upstream face and the upstream retainer, under the ring through grooves in the valve body, and out of the valve through grooves in the downstream stop. When closed (FIG. 1b), the ring's upstream face's tapered surface contacts the upstream stop's complimentary surface, forming a sealing surface and blocking flow through the valve. Thus, when the ring is in its downstream position, the valve's flowpath is open; when upstream, the ring blocks the flowpath. Existing ring valves require a land length for sealing. The greater the length of this sealing surface, or land length, the better the seal. The ring/valve body assembly is attached to the downstream end of a feed screw. As the screw rotates, it feeds material into the non-return valve. The material forces the ring to its open position, allowing the material to flow through the valve and into an accumulation volume.
Ring valves do not rely upon a back leakage-induced pressure drop to close the valve. Instead, ring valves rely upon a very tight clearance between the ring's outer diameter and the barrel's inner diameter to create a friction force that holds the ring in place as the valve body strokes forward during injection, forcing the shot of plastic out of the valve and into the mold. In other words, when the valve body strokes forward, the friction force tends to hold the ring in place relative to the stationary barrel. As the valve moves forward within the barrel, the ring moves upstream relative to the valve body, blocking the material flowpath. The friction force between the ring and the barrel also opposes the ring's downstream travel when the screw begins to rotate and feeds material into the valve body. This reduces the ring's downstream velocity relative to the valve body and the resultant force with which it impacts the downstream retainer.
The friction between the ring and the barrel also creates a force opposing the rotation of the ring along with the valve body when the screw rotates and feeds material through the valve body. Thus, the valve body tends to rotate inside the ring, which is held almost stationary against the barrel's inner surface by the friction force. Because the valve's downstream retainer rotates with the valve body, and the ring's downstream surface contacts the downstream retainer, a rotational friction force results between the ring's downstream seating surface and the retainer. This results in wear of the ring--shortening the ring--and of the retainer. The resulting erosion of these surfaces increases the distance that the ring must travel upstream during injection to cover the valve inlets, increasing valve closure time and valve leakage prior to closing. As cumulative wear between the ring's outer surface and the barrel's inner surface increases the clearance between the ring and the barrel, the friction force decreases and dynamic forces created by the material flow become the primary forces shutting the valve. However, these same flow-induced dynamic forces can cause the ring to open too quickly and impact with great velocity upon the ring's downstream retaining device. Moreover, the decreased friction between the ring and barrel allows the ring to impact the downstream retainer with greater velocity. Finally, while a longer land length, or sealing surface, minimizes leakage through the valve, the increased surface area of the ring's upstream surface results in a greater force (for a given pressure) upon the ring in the downstream direction when flow-induced dynamic forces close the valve. Over the valve's life, this impact can cause increased wear and even premature valve failure. To minimize ring and downstream stop erosion, prior art valves are often necessarily constructed of very hard materials, such as H-13 tool steel, with specially hardened retaining surfaces. These materials are expensive; moreover, their increased brittleness increases the chances of catastrophic failure due to repeated impact.
With both ball and ring type valves, particulate contamination, poor alignment, and wear of the sealing surface worsens back leakage--and consequent injection mass variation--by preventing a perfect seal and allowing back leakage through the valve even after valve closure. Therefore, a need exists for a non-return valve that always furnishes the same shot size regardless of plastic, fillers, contamination, product produced, or wear. This valve should be designed to allow its incorporation into existing injection molding machines or any other device which utilizes a non-return valve. This valve should not depend solely upon leakage through the valve or barrel to ring friction to generate the force necessary to move the valve to its closed position. Furthermore, this valve should be designed so that particles can never impair the seal. Finally, the valve should limit the impact of the closing ring against its retaining devices.